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How To Understand True Pump Horsepower - Up Rated vs Full Rated

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Have you ever wondered about the differences between full rate and up-rated motor designations?  No worries because while these designations have led to a decades-old mystery of sorts confusing even the most experienced and knowledgeable distribution, retail, and installation professionals, the remedy is quite simple.

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Steps

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Step 1

Instead of being treated to such classic yet equally vacuous answers “It’s always been that way,” or “Who knows,” a simple equation for determining total horsepower (THP), or true horsepower, will equip you to easily discern between the ratings and make substitutions without hesitation.

Step 2

A little background is probably beneficial at this point to help explain the “why” behind the use of the designations.  Historically, home water systems in the 1940’s were predominately found in more rural areas of the country where electrical service was not always consistent and low voltage situations were common.  For this reason, NEMA standards for service factor were set higher especially on lower horsepower motors for pump applications.  The higher service factor resulted in more start and run torque allowing the pumps to operate under low voltage conditions.

Step 3

The leisure water industry began to grow leading to pumps and motors being adapted to swimming pool and spa applications.  Original equipment manufacturers began to apply larger impellers using the full service factor horsepower – simply different terminology for total horsepower – in order to move more water (GPM) for each rating.  These developments led to high service factor (full rate) and low service factor (up-rated) models offered to the market.

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Step 4

Eventually regional preferences developed for either full rated or up-rated motors offered on new pump systems as well as replacements.  For illustrative purposes then, Florida remains a largely up-rated motor market today. Still, no region is exclusively full rated or up-rated – leading pool professionals to needlessly grapple with the confusing designations when presented with motors that are not equivalent even though the nameplate horsepower appears to indicate otherwise.

Step 5

For example, the full rate SQ1102 and the up-rated USQ1102 are both 1.0 nameplate horsepower square flange replacement motors.  As discussed in the March 2013 issue, the SQ1102 actually has a total horsepower (THP) of 1.65, compared to a 1.25 THP for the USQ1102.

Step 6

The solution – multiply the nameplate horsepower by the nameplate service factor to determine the total horsepower.  Make it a habit of determining the total horsepower before selecting a replacement motor, especially if going from one rating designation to another.  Choose a replacement with an equivalent total horsepower or slightly above that of the motor being replaced. 

Step 7

Horsepower (HP) x Service Factor (SF) = Total Horsepower (THP)

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(1 to 15 of 15)

 Posted: 7/11/2020 

I moved into a home with an in-ground pool that is about 21,260 gallons and has a 2 HP motor with a 1.1 SF. It had several leaks around the pump housing and some of the plumping which I fixed. It wasn’t until then that the PSI on my filter shot up to 30. Based on the numbers, should I have a .5 HP motor?
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InyoPools Product Specialist  Posted: 8/25/2020 

A half horsepower is not enough for a pool of that size. Are you sure the spike in PSI was not due to a dirty filter? If your pump was in that bad of shape, it is a possibility the filter hadn't been cleaned for a while. When you fixed the leaks, you also correct some flow inefficiency problems in the system, allowing the pump to reach its normal operating pressure. Also, if the pump and filter have been there for a while, I presume they were working well enough for the previous owner to keep it as it is. I'd work on cleaning the filter and then figure out the pump if the pressure problems persist.
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 Posted: 7/11/2020 

in looking at a replacement for ST 1072 a 3/4 HP service rated 1.5 Century motor I find a 3/4 hp motor that is labeled SQ 1072 but the service factor is labeled '1.50 - 1.75 and 1.65' Why two numbers and why a range??
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InyoPools Product Specialist  Posted: 9/24/2020 

Those two motors are not compatible with one another. The ST is a round flange motor, and the SQ is a square. If you need a replacement motor, the correct one is the ST1072
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 Posted: 3/11/2020 

I replaced a Pentair model 11511 motor (1/2 HP SF 1.9 = SFHP 0.95) with a Pentair 196237 (355018S) which is 1/2 HP but pump label SF and SFHP are blank. It has burned out within weeks. I assume that the true SF on this new motor is 1 or less (so legally not required to state it on the label) so my SFHP dropped from .95 to .5 or less, overloading the motor. Does this make sense, are my assumptions correct? Many thanks.
 Reply

InyoPools Product Specialist  Posted: 3/12/2020 

The 355018S is a full-rated .5 HP motor, so it should work. It is a standard efficiency instead of your original energy-efficient motor, which is the only difference. You may have received a dud motor, or there was an issue with the circuit that needs troubleshooting.But you are correct in that; a .5 HP uprated motor is not a straight replacement for a .5 HP full-rated. To make the .5 HP uprated work on the full rated pump, you would need to change the impeller, and maybe even the diffuser.
 Reply

 Posted: 11/15/2018 

So, would this process primarily be used to find a suitable sub in cases where your original motor is unavailable?
 Reply

InyoPools Product Specialist  Posted: 11/16/2018 

Hello Terry - Yes, this can be used to find a suitable replacement motor if the original is not available. The total HP of the replacement needs to be equal to or greater than the current total HP. Total HP is calculated by multiplying the HP and Service Factor. The other specifications that need to match are frame and voltage. 
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 Posted: 8/8/2018 

Hello Danny - there should not be much of a difference either way. The main concern should be matching the product of Listed HP and Service Factor, called Total Horsepower.
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 Posted: 8/7/2018 

If possible should one choose a motor (pump) with a S.F. closer to 1 on the premise that the motor is not operating in "overload" condition and will result in less heat and longer service life of the motor?
 Reply

Anonymous  Posted: 2/3/2020 

This is the same question I have. I’m replacing my 1.5HP motor and the salesman wants me to purchase the 1.0HP w/ a higher SF.
 Reply

InyoPools Product Specialist  Posted: 2/10/2020 

That is the basis of this entire guide, but the foundation is this simple equation, Horsepower (HP) x Service Factor (SF) = Total Horsepower (THP) - from Step 7.If you input the HP and service factors of the two motors, and the products of the two calculations are within .15 of one another, they are comaptible.
 Reply

 Posted: 6/30/2016 

Very helpful. You almost lost me in the technical language early on but I reminded myself that you were describing context. In the end it was totally clear. Thank you.
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 Posted: 4/3/2016 

Very helpful. Thank you!
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 Posted: 9/7/2015 

This was a clear and organized explanation. Thanks.
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